In recent years, surface acoustic wave devices have found wide application in communication devices such as a motor vehicle telephones as circuit elements of resonator filters, signal processing delay lines, etc. For example, FIG. 13 shows a surface acoustic wave device comprising interdigital electrodes 2 and latticelike reflectors 3, 3 which are formed on the surface of a piezoelectric substrate 1. The device converts electric signals to surface acoustic waves and vice versa.
Surface acoustic waves are surface waves which literally propagate along the surface of an elastic body, and the energy thereof is not radiated into the substrate. A plurality of modes of excitation have been discovered as such surface acoustic waves. For example, already known are the Rayleigh wave, Sezawa wave, Love wave, electroacoustic wave, etc.
In the Rayleigh and Sezawa waves, predominant are both a longitudinal wave component having a displacement in the same direction as the direction of propagation and a shear wave component having a displacement depthwise of the substrate. On the other hand, predominant in the Love wave and the electroacoustic wave is a shear wave component having a displacement in parallel to the substrate surface and perpendicular to the propagation direction. While three kinds of bulk waves, i.e., "slow shear wave", "fast shear wave" and "longitudinal wave" are present in the piezoelectric substrate, the surface acoustic waves propagate at a phase velocity lower than that of the "slow shear wave".
Also known are elastic waves which propagate along the surface of an elastic body while radiating energy depthwise of the body. These waves are called quasi surface acoustic waves or leaky surface acoustic waves. The quasi surface acoustic wave initially discovered comprises a predominant shear wave component having a displacement in parallel to the substrate surface and perpendicular to the propagation direction, and is intermediate between the "slow shear wave" and the "fast shear wave" in phase velocity.
Quasi surface acoustic waves having a predominant longitudinal wave component are discovered in recent years one after another (see JP-A-112763/1994 and proceedings of the 15th Symposium on Fundamentals and Applications of Ultrasonic Electronics, 1994, pp. 185-186). These quasi surface acoustic waves having a predominant longitudinal wave component are intermediate between the "fast shear wave" and the "longitudinal wave in phase velocity".
On the other hand, there is a case wherein bulk waves propagating along and close to the surface of a substrate are excited by interdigital electrodes and detected by other interdigital electrodes on the same substrate. Such bulk waves are termed surface skimming bulk waves. It is thought that there are three kinds of surface skimming bulk waves in corresponding relation with the usual bulk waves. However, mainly handled at present is the surface skimming bulk wave which comprises a predominant shear wave component having a displacement in parallel to the substrate surface and perpendicular to the propagation direction.
The characteristics of elastic waves include acoustic velocity, propagation loss, temperature characteristics of delay time and electromechanical coupling coefficient. These characteristics relate directly to the design parameters of the circuit to which the surface acoustic wave device is applied.
The period (center-to-center distance) of the electrode fingers of interdigital electrodes or latticelike reflectors has a value which is 1/2 of the wavelength .lambda. of elastic waves, so that at a constant frequency, the smaller the acoustic velocity, the smaller is the wavelength and the more difficult are the electrodes to fabricate. It is therefore desired that the acoustic velocity be greater.
The resonance sharpness of surface acoustic wave resonators and the insertion loss of surface acoustic wave filters are dependent directly on the propagation loss of surface acoustic waves. For this reason, the propagation loss should preferably be small.
The high-frequency devices for use in mobile communication are used at a frequency specified by the standard. Accordingly, it is not desirable that the frequency varies with variations in temperature. The temperature coefficient of delay time should preferably be small, therefore.
The electromechanical coupling coefficient represents a capacity to convert the energy of an input electric signal into the energy of surface acoustic waves. When the interdigital electrodes have a sufficiently increased number of electrode fingers, elastic waves of desired energy can be excited even if the electromechanical coupling coefficient is small, whereas the interdigital electrodes then have an increased electrical capacity, which presents difficulty in impedance matching with an external circuit, necessitating an additional matching circuit for impedance matching. Further it is known that the number of electrode fingers of interdigital electrodes is approximately in inverse proportion to the operating frequency range of the surface acoustic wave device, such that an increase in the number of electrode fingers limits the realizable characteristics to a narrow frequency range. Accordingly, the electromechanical coupling coefficient is preferably great.
Already known are substrate conditions (e.g., the relationship between the crystal axis and the direction of propagation of surface acoustic waves) and electrode conditions (e.g., the center-to center distance and film thickness of the electrode fingers) for improving the foregoing characteristics in connection with elastic waves (such as the Rayleigh wave and Sezawa wave) which comprise two predominant components, i.e., a shear wave component having a depthwise displacement and a longitudinal wave component, and elastic waves (such as the electroacoustic wave, Love wave, quasi surface acoustic wave of the shear wave type and surface skimming bulk wave of the shear wave type) which comprise a predominant shear wave component having a displacement in parallel to the surface and perpendicular to the propagation direction (Proceedings of the 1994 IEICE (Institute of Electronics, Information and Communication Engineers) Spring Conference, "A-438", "A-437", "A-438", Japanese Journal of Applied Physics, vol. 29 (1990) Supplement 29-1, pp. 119-121, Japanese Journal of Applied Physics, vol. 30 (1991) Supplement 30-1, pp. 143-145, etc.).
However, the requirements of the substrate and electrodes for improving the above characteristics still remain to be clarified for surface acoustic waves wherein the longitudinal wave component predominates over the shear wave component (longitudinal wave-type surface acoustic waves), quasi surface acoustic waves wherein the longitudinal wave component predominates over the shear wave component (longitudinal wave-type quasi surface acoustic waves) and surface skimming bulk waves wherein the longitudinal wave component predominates over the shear wave component (longitudinal wave-type surface skimming bulk waves).